How to create powershell cmdlet (C#) output parameters

Question

There are several examples in the common parameters, like ErrorVariable, InformationVariable;

get-item foad: -ev foo
$foo

"get-item" will create and set the value for $foo to an instance of ErrorRecord. How can I create my own parameters that do something similar?

Basically, I'm creating a cmdlet that uses WriteObject() to write data to the pipeline, but then I also have some additional information that I want to allow users to access - essentially out of band data - that isn't part of the pipeline.

An example of an out parameter in C#:

public class ExampleCode
{
    public int GetMyStuff(int param1, out string someVar)
    {
        someVar = "My out-of-band result";
        return param1 + 1;
    }

    public static void RunMe()
    {
        ExampleCode ex = new ExampleCode();
        string value;
        int result = ex.GetMyStuff(41, out value);

        Console.WriteLine($"result is {result}, OOB Data is {value}");
    }
}

I'm looking for how to convert "GetMyStuff()" into a powershell cmdlet.

[Cmdlet(VerbsCommon.Get, "MyStuff")]
public class ExampleCmdLet : PSCmdlet
{
    [Parameter(Mandatory = false)] int param1;
    [Parameter(Mandatory = false)] string someVar; // How to make this [out] ?
    protected override void ProcessRecord()            
    {
        someVar = "My out-of-band result";
        WriteObject(param1 + 1);
    }
}

mklement0 · Accepted Answer · 2023-10-07 11:06:14Z

2

You're looking to set a PowerShell variable, not a .NET variable.

Accessing PowerShell variables requires accessing them via the caller's session state.

In System.Management.Automation.PSCmdlet-derived cmdlets you can set variables via this.SessionState.PSVariable.Set(<varName>, <value>):

# Compile a Get-MyStuff cmdlet and import it into the current session.
Add-Type -TypeDefinition @'
using System.Management.Automation;

[Cmdlet(VerbsCommon.Get, "MyStuff")]
public class ExampleCmdLet : PSCmdlet
{
    [Parameter()] public int Param1 { get; set; }
    [Parameter()] public string SomeVar { get; set; }

    protected override void ProcessRecord()
    {

        // Assign to the $SomeVar variable in the caller's context.
        this.SessionState.PSVariable.Set(SomeVar, 42);

        WriteObject(Param1 + 1);
    }

}
'@ -PassThru | % Assembly | Import-Module                                                                           #'

# Call Get-MyStuff and pass the *name* of the 
# variable to assign to, "targetVariable", which sets
# $targetVariable:
Get-MyStuff -Param1 666 -SomeVar targetVariable
# Output the value of $targetVariable
$targetVariable

The above yields:

667  # Output from the cmdlet, via WriteObject()
42   # Value of $targetVariable, set by Get-MyStuff

edited Oct 7, 2023 at 11:06

answered Feb 11, 2020 at 3:38

mklement0

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1 Comment

FSCKur Over a year ago

To expand on this answer - accepting a string variable name is the canonical way to do this in PowerShell - e.g. Tee-Object -Variable MyVar creates $MyVar, or the automatic ErrorVariable parameter on all cmdlets. If you're nice, you'll support the + prefix to append to an existing ArrayList. An alternate way to do it is to accept a parameter of type [ref], but that's not very powershelly.

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